To meet the growing demand of high rate wireless data transmission, transmitters with large numbers of antennas and serving a multitude of users (massive MIMO systems) are currently advocated for future wireless cellular systems. One possible application of such a scenario is the downlink (DL) of the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE)/LTE-Advanced. One of the main challenges in multi-input multi-output (MIMO) and massive MIMO operation is acquiring accurate channel state information (CSI) at the base station (BS) to perform beamforming or precoding operations. In LTE systems, this is performed by sending predefined signals—pilots or pilot sequences—that are used for channel estimation.
Future 5G systems will utilize frequency spectrum efficiently and will move towards a frequency reuse of 1. This implies the pilots of a desired user will suffer from inter-cell pilot interference when the pilots of users in neighboring cells are scheduled on the same time and frequency resources as those of the pilots of the desired user. This interference degrades the CSI quality at the BS. Designing pilot sequences that are orthogonal across any number of cells is fundamentally not possible. Therefore, pilot sequences are usually limited to be orthogonal within one cell and one has to devise new solutions to mitigate inter-cell pilot interference.
The inter-cell pilot interference problem is especially severe for cell-edge users, since the power of interfering pilots is comparable to the power of desired pilots.
One possible scenario where inter-cell pilot interference is detrimental is in time-division-duplex (TDD) mode. Here, due to channel reciprocity, the transmitter can estimate the CSI of desired users based on pilots sent by these users in the uplink (UL) and use these estimates to perform beamforming or precoding in the DL. However, UL inter-cell pilot interference deteriorates the CSI quality at the BS and leads to mismatched precoders which deteriorate the DL performance in TDD mode. Another scenario that applies to both TDD and frequency-division-duplex (FDD) mode is in the context of UL data transmission. Here, UL inter-cell pilot interference deteriorates the quality of the UL CSI necessary at the BS to decode/demodulate the symbols sent by desired users.
LTE uses following procedures to randomize interference to other cells. Firstly according to a sequence-group hopping, the sequence used in a given cell depends on a group hopping pattern and a sequence shift pattern. In this respect, there are 17 hopping patterns/groups and 30 shift patterns. Further on, a cluster of up to 30 cells can belong to the same hopping group, and cells within the cluster are differentiated via the shift pattern. Secondly according to a cyclic shift hopping, a cell specific cyclic shift is added on top of UE specific cyclic shifts. It is important to note that LTE thereby is only able to randomize interference to other cells.